Semiconductor device and method of fabricating the same

ABSTRACT

A semiconductor device includes an isolation layer defining an active region formed in a semiconductor substrate. A first recessing process is performed on the isolation layer to expose edge portions of the active region. A first rounding process is performed to round the edge portions of the active region. A second recessing process is performed on the isolation layer. A second rounding process is performed to round the edge portions of the active region.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This is a divisional application of U.S. application Ser. No.11/931,571, filed Oct. 31, 2007 now U.S. Pat. No. 7,833,875, which is acontinuation-in-part application of U.S. patent application Ser. No.11/149,396, now U.S. Pat. No. 7,342,280 filed Jun. 9, 2005, which is adivisional application of U.S. patent application Ser. No. 10/446,970,now U.S. Pat. No. 6,913,969 filed on May 28, 2003, the disclosures ofwhich are herein incorporated by reference in their entirety.

This application also claims the benefit of Korean Patent ApplicationNo. 10-2007-0038327, filed on Apr. 19, 2007, in the Korean IntellectualProperty Office, the disclosure of which is herein incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method offabricating the same.

2. Description of Related Art

As semiconductor device integration increases, a size of an activeregion on which a channel is formed is reduced. Semiconductor deviceshaving small active regions may exhibit high leakage currents and lowdriving performances. For example, when a channel length is reduced, ashort channel effect can occur, and when a channel width is reduced, adriving current can be decreased.

Accordingly, there is a need to increase a channel area in highlyintegrated semiconductor devices. For example, the active region canhave a larger surface area by forming a protrusion in the active regionwith respect to an isolation layer. In the active region, sidewalls aswell as an upper surface can be used as a channel, and thus, the drivingperformance of the semiconductor device can be increased.

When the sidewalls of an active region are used as a channel, anelectric field can be enhanced in an edge portion of the active region.The electric field increases as the curvature radius of the edge portionof the active region decreases. A threshold voltage of a semiconductordevice can be changed according to the profile of the edge portion ofthe active region, and the threshold voltages between semiconductordevices respectively fabricated using a single wafer or a single batchhave a wide distribution range. The wide distribution range of thethreshold voltages decreases the reliability of the semiconductordevice.

Furthermore, the profile of the edge portion of the active region canaffect the programming characteristics of a non-volatile memory device.When the electric field is enhanced in the edge portion of the activeregion, more tunneling effects of electrons or holes may occur in theedge portion of the active region. As a result, a tunneling insulatinglayer disposed on the edge portion of the active region deteriorates,and the non-volatile memory device has low durability and lowhigh-temperature reliability.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, an isolation layermay be recessed from the surface of a semiconductor substrate. An activeregion may be defined in the semiconductor substrate by the isolationlayer, the active region protruding upward with respect to the isolationlayer. A curvature radius of edge portions of the active region may bein the range from about ⅓ to about ½ of the width of an upper portion ofthe active region.

According to another embodiment of the present invention, a method offabricating a semiconductor device includes forming an isolation layerdefining an active region in a semiconductor substrate. A plurality ofrecessing processes may be performed on the isolation layer to exposeedge portions of the active region. A plurality of rounding processesmay be performed to round the edge portions of the active region.

The rounding processes and the recessing processes may be performedalternately.

According to another embodiment of the present invention, at least onerounding process from among a plurality of rounding processes mayinclude etching edge portions of the active region.

According to another embodiment of the present invention, at least onerounding process from among a plurality of rounding processes mayinclude oxidizing edge portions of the active region.

According to another embodiment of the present invention, a method offabricating a semiconductor device includes forming an isolation layerdefining an active region in a semiconductor substrate. A firstrecessing process may be performed on the isolation layer to expose edgeportions of the active region. A first rounding process may be performedto round the edge portions of the active region. A second recessingprocess may be performed on the isolation layer. A second roundingprocess may be performed to round the edge portions of the activeregion.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent by describing in detailexemplary embodiments thereof with reference to the attached drawings inwhich:

FIG. 1 is a sectional view of a semiconductor device according to anembodiment of the present invention;

FIGS. 2 through 6 are sectional views illustrating a method offabricating a semiconductor device according to an embodiment of thepresent invention;

FIG. 7 is a sectional view illustrating a method of fabricating asemiconductor device according to another embodiment of the presentinvention;

FIG. 8 is a sectional view illustrating a method of a semiconductordevice according to another embodiment of the present invention;

FIG. 9 is a transmission electron microscopic (TEM) image of a sectionalview of a semiconductor device fabricated according to a comparativeexample of the present invention;

FIG. 10 is a TEM image of a sectional view of a semiconductor devicefabricated according to an example of the present invention;

FIG. 11 is a graph illustrating the distribution of threshold voltagesof a semiconductor device prepared according to the comparative exampleof the present invention; and

FIG. 12 is a graph illustrating the distribution of threshold voltagesof a semiconductor device prepared according to an example of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown. The invention may, however, be embodied in manydifferent forms and should not be construed as being limited toembodiments set forth herein; rather, embodiments are provided so thatthis disclosure will be thorough and complete, and will fully convey theconcept of the invention to those skilled in the art. In the drawings,the thicknesses of layers and regions are exaggerated for clarity. Likereference numerals in the drawings denote like elements, and thus theirdescription will be omitted.

A semiconductor device according to embodiments of the present inventionmay include memory devices and/or logic devices.

FIG. 1 is a sectional view of a semiconductor device according to anembodiment of the present invention.

Referring to FIG. 1, an active region 115 may be defined in asemiconductor substrate 105 by an isolation layer 110. The semiconductorsubstrate 105 includes, for example, silicon (Si), germanium (Ge), orsilicongermanium (SiGe). The active region 115 may be used to form anactive device such as a transistor or a capacitor. The isolation layer110 may electrically isolate the active device. The isolation layer 110may include an insulating layer, for example, an oxide layer or nitridelayer.

The isolation layer 110 is, for example, a shallow trench isolation(STI) layer. The isolation layer 110 may be formed by filling a trenchextending to an inner portion of the semiconductor substrate 105. Theisolation layer 110 may be recessed from a surface of the semiconductorsubstrate 105 to a predetermined depth. As a result, edge portions E ofthe active region 115 may be exposed by the isolation layer 110. Theisolation layer 110 may be recessed to expose a part 120 b′ of sidewalls120 of the active region 115.

The surface of the active region 115 exposed by the isolation layer 110may be used as a channel that is a conductive passage for charges. Agate electrode (not shown) may cover the exposed surface of the activeregion 115. The active region 115 protruding with respect to theisolation layer 110 may have a different structure from a planar-typestructure, that is, a fin-type structure. Accordingly, the structure ofthe active region 115 may provide a greater driving current than theplanar structure, and thus, the driving performance of a semiconductordevice may be improved.

The edge portions E of the active region 115 may be rounded. Such arounded shape may substantially prevent enhancement of an electric fieldgenerated from the gate electrode at the edge portions E of the activeregion 115. As a result, threshold voltage irregularity due to irregularelectron fields at the edge portions E of the active region 115 can bedecreased, and reliability of a semiconductor device can be improved.

For example, a curvature radius R of the edge portions E of the activeregion 115 may be in the range from about ⅓ to about ½ of the width W ofan upper portion of the active region 115. When the curvature radius Ris smaller than about ⅓ of the width W, an electric field enhancementdecrease effect is small, and thus, threshold voltages may be irregular.When the curvature radius R is about ½ of the width W, the upper portionof the active region 115 is rounded and has a curvature radius, and ahigh electric field enhancement decrease effect can be obtained. Whenthe curvature radius R is greater than about ½ of the width W, the upperportion of the active region 115 may have a sharp pointed part, andthus, an electric field enhancement may occur.

When the semiconductor device according to an embodiment of the presentinvention is a non-volatile memory device, an electric field enhancementdecrease at the edge portions E of the active region 115 may contributeto high reliability of a tunneling insulating layer (not shown) of thenon-volatile memory device, where a local electric field enhancement maycause tunneling of charges in a portion of the tunneling insulatinglayer on the active region 115. In addition, the active region 115 has alarger surface area and a charge storage layer formed on the activeregion 115 may also have a larger area. For the active region 115 havinga larger surface area, the charge storage layer may store more charges,and reliability of multi-bit operation using a local charge trap can beimproved.

FIGS. 2 through 6 are sectional views illustrating a method offabricating a semiconductor device according to an embodiment of thepresent invention.

Referring to FIG. 2, an isolation layer 110 may be formed on asemiconductor substrate 105 to define an active region 115. For example,a trench (not shown) is formed in the semiconductor substrate 105, andfilled with an insulating layer. For example, the insulating layer mayinclude an oxide layer or a nitride layer. The insulating layer may beplanarized using an etch-back method or a chemical mechanical polishing(CMP) method. The planarizing process can be performed using aprotective layer (not shown) disposed on the active region 115 as a stoppoint, e.g., an etch stop. At this time, the isolation layer 110 maysurround the sidewalls 120 of the active region 115 and may protrudefrom the surface of the semiconductor substrate 105 to a predeterminedheight.

Referring to FIG. 3, a first recessing process is performed to recessthe isolation layer 110 so that edge portions E of the active region 115are exposed by the isolation layer 110. For example, the isolation layer110 can be recessed to expose a first portion 120 a of the sidewalls120. The height of the first portion 120 a can be adjusted according toa number of rounding processes and a rounding efficiency.

For example, the first recessing process of the isolation layer 110 canbe performed using a wet etching method or a dry etching method. Whenthe isolation layer 110 is an oxide layer, the wet etching may beperformed using a HF solution.

Referring to FIG. 4, a first rounding process is preformed to round theedge portions E of the active region 115. For example, the firstrounding process may be performed by partially etch the active region115. The edge portions E of the active region 115 can be rounded sincethe edge portions E of the active region 115 having a wide surface areaare more quickly etched than other portions of the active region 115.The width of a first portion 120 a′ may be smaller than the width of afirst portion 120 a (see FIG. 3) due to the first rounding process.

For example, the active region 115 can be isotropically and/oranisotropically etched. The isotropic etching may be performed using awet etching method or a chemical dry etching (CDE) method. For example,the wet etching method may use a mixture (SC1) solution of NH₄OH, H₂O₂,and H₂O. The anisotropic etching can be performed using a plasma dryetching method. The rounding process can be performed using theanisotropic etching according to the shape of the active region 115 andthe concentration of radicals in plasma.

Referring to FIG. 5, a second recessing process is performed on theisolation layer 110. For example, the isolation layer 110 can berecessed to expose a second portion 120 b of the sidewalls 120 of theactive region 115. The height of the second portion 120 b may be largerthan the height of the first portion 120 a. The second recessing processmay be performed on the isolation layer 110 using, for example, a wetetching method or a dry etching method.

Referring to FIG. 6, a second rounding process is performed to round theedge portions E of the active region 115. For example, the secondrounding process can be performed by partially etching the active region115. The edge portions E of the active region 115 can be rounded sincethe edge portions E of the active region 115 having a wide surface areaare more quickly etched than other portions of the active region 115.The width of a second portion 120 b′ may be smaller than the width ofthe first portion 120 a′ (see FIG. 4) due to the second roundingprocess. For example, the active region 115 can be isotropically and/oranisotropically etched, as described with reference to the firstrounding process.

The isolation layer 110 is gradually recessed through the first andsecond rounding processes. The first portion 120 a of the sidewalls 120of the active region 115 is exposed, and then the second portion 120 bof the sidewalls 120 of the active region 115 is exposed. Accordingly,the first portion 120 a exposed through the first recessing process maybe etched twice through first and second rounding processes, and a newlyexposed portion of the sidewalls 120 of the active region 115 throughthe second recessing process may be etched once. As a result, the widthof the active region 115 may be increased in a direction toward theisolation layer 110. Accordingly, in the rounding processes, a decreasein the surface area of the active region 115 due to a decrease in thewidth of the active region 115 can be substantially prevented.

Through the first and second rounding processes, the edge portions E canbe sufficiently rounded. For example, a curvature radius R of the edgeportions E of the active region 115 may be in the range from about ⅓ toabout ½ of a width of an upper portion of the active region 115.Accordingly, through first and second recessing processes and first andsecond rounding processes, the edge portions E of the active region 115are sufficiently rounded and a decrease in the width and surface area ofthe active region 115 can be substantially prevented.

Subsequently, a semiconductor device can be completely fabricated usinga method of fabricating a semiconductor device known to those ofordinary skill in the art.

FIG. 7 is a sectional view illustrating a method of a semiconductordevice according to another embodiment of the present invention. Themethod of fabricating a semiconductor device is the same as the methodaccording to the previous embodiment described with reference to FIGS. 2through 6, except that the first rounding process is modified. FIG. 7shows a sectional view of a semiconductor device fabricated by modifyingthe first rounding process described with reference to FIG. 4. Thedescription of elements in the FIG. 7 similar to the elements in FIGS. 2through 6 will not be repeated.

Referring to FIG. 7, a first rounding process is performed to oxidize asurface of the active region 115 exposed by the isolation layer 110 toform a sacrificial layer 123. The width of the first portion 120 a″ ofthe sidewalls 120 may be smaller than the width of the edge portion 120a (see FIG. 3) due to the first rounding process. Portions of thesacrificial layer 123 formed in edge portions E of the active region 115to which oxygen is supplied may be thick, and thus, the edge portions Eof the active region 115 can be rounded by removal of the sacrificiallayer 123.

FIG. 8 is a sectional view illustrating a method of a semiconductordevice according to another embodiment of the present invention. Themethod of fabricating a semiconductor device is the same as the methoddescribed with reference to FIGS. 2 through 6, except that the secondrounding process is modified. That is, FIG. 8 shows a sectional view ofa semiconductor device fabricated by modifying the second roundingprocess described with reference to FIG. 6. Accordingly, the descriptionof elements similar to the elements described with reference to FIGS. 2through 6 will not be repeated.

Referring to FIG. 8, the second rounding process is performed to oxidizea surface of an active region 115 exposed by an isolation layer 110,thereby forming a sacrificial layer 133. As a result, the width of thesecond portion 120 b″ of the sidewalls 120 may be smaller than the widthof the first portion 120 a′ (see FIG. 4) of the sidewalls due to thesecond rounding process. Portions of the sacrificial layer 133 formed inedge portions E of the active region 115 to which oxygen is supplied maybe thick, and thus, the edge portions E of the active region 115 can berounded by removal of the sacrificial layer 133.

In the method of fabricating a semiconductor device described withreference to FIGS. 2 through 6, the first rounding process describedwith reference to FIG. 4 and the second rounding process described withreference to FIG. 6 may be modified to the first rounding processdescribed with reference to FIG. 7 and the second rounding processdescribed with reference to FIG. 8, respectively.

In previous embodiments of the present invention, two recessingprocesses and two rounding processes are performed. However, the numberof the recessing and rounding processes are not limited thereto. Forexample, a plurality of recessing processes and a plurality of roundingprocesses can be performed alternately. A plurality of recessingprocesses may be understood with reference to the first and secondrecessing processes described above. A plurality of rounding processesmay be understood with reference to the first and second roundingprocesses described above. The number of recessing and roundingprocesses may be limited in consideration of the manufacturing costs.

FIG. 9 is a transmission electron microscopic (TEM) image of a sectionalview of a semiconductor device fabricated according to a comparativeexample of the present invention, and FIG. 10 is a TEM image of asectional view of a semiconductor device fabricated according to anexemplary embodiment of the present invention. In forming the devicedepicted in FIG. 10, two rounding processes were performed to fabricatea semiconductor device; whereas, in the comparative example, only onerounding process was performed to fabricate a semiconductor device. InFIGS. 9 and 10, the semiconductor device was a NAND-type non-volatilememory device.

Referring to FIG. 9, in the comparative example, edge portions E1 of anactive region 115 a are not rounded. However, referring to FIG. 10, inthe example, edge portions E2 of an active region 115 b are rounded tohave a curvature radius.

FIG. 11 is a graph illustrating the distribution of threshold voltagesof a semiconductor device fabricated according to the comparativeexample of FIG. 9, and FIG. 12 is a graph illustrating the distributionof threshold voltages of a semiconductor device fabricated according tothe example of FIG. 10. With respect to an NAND structure, thedistribution of threshold voltages was obtained by measuring thethreshold voltage of memory transistors except for two memorytransistors located in both ends of the NAND structure.

Referring to FIG. 11, the memory device fabricated according to thecomparative example has the threshold voltage distribution of about 3.1V. Referring to FIG. 12, the memory device fabricated according to theexample of FIG. 10 has the threshold voltage distribution of about 2.5.Accordingly, the memory device fabricated according to the example ofFIG. 10 has narrower threshold voltage distribution than the memorydevice fabricated according to the comparative example of FIG. 9. Suchan improvement on the threshold voltage distribution in the example ofFIG. 10 may result from a uniform electric field distribution due tohigh rounding effects.

A semiconductor device according to embodiments the present inventionhas a high driving performance due to a large active region, and highreliability due to low electric field enhancement in edge portions ofthe active region.

A semiconductor device according to embodiments of the present inventioncan be a non-volatile memory device with a tunneling insulating layerhaving high durability and high-temperature reliability.

According to a method of fabricating a semiconductor device, the surfaceof an active region can be efficiently widened and high rounding effectscan be obtained, by repeatedly using a recessing process and a roundingprocess.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention.

1. A semiconductor device comprising: an isolation layer recessed from asurface of a semiconductor substrate; and an active region defined bythe isolation layer in the semiconductor substrate, the active regionprotruding upward with respect to the isolation layer, wherein acurvature radius of edge portions of the active region is in the rangefrom about ⅓ to about ½ of a width of an upper portion of the activeregion.
 2. The semiconductor device of claim 1, wherein the curvatureradius of edge portions of the active region is about ½ of a width ofthe upper portion of the active region.
 3. The semiconductor device ofclaim 1, wherein the width of the active region increases in a directiontoward the isolation layer.